Various implantable medical devices, often referred to as implantable infusion pumps, are used for dispensing controlled volumes of a drug within a patient's body. Such devices can provide a convenient vehicle for delivering the drug over an extended period of months to years. Such drugs can treat a number of indications including cancer, chronic pain, anti-coagulation, and spasticity.
For infusion pumps that utilize an internal reservoir to hold a supply of drug to be delivered to a patient, periodic refilling of the drug reservoir may be necessary to replenish a depleted drug supply. For example, a syringe needle can be used to inject an additional volume of drug through a septum-covered port connected to the reservoir. Accordingly, timely information regarding the quantity of drug in the reservoir can be a useful feature. Control units associated with regulated operation of an infusion pump can provide data regarding the amount of drug in a reservoir based on the programmed, and expected, operation of the pump by the control unit. Such indirect information, however, can be misleading due to a number potential operating errors (e.g., obstructions or leakages in the drug delivery system).
Though sensors can also be used to determine the quantity of drug remaining in a drug reservoir, sensing devices suitable for use with an implantable infusion pump are still subject to operational constraints that limit their accuracy. For example, measurements from some sensors have a temperature-dependent component that affects the absolute reading provided by the sensor—typically, such sensors are only accurate where the temperature is controlled to within about ±0.1° C. A normal patient's temperature can vary on the order of ±0.4° C., and a patient who is ill can have their temperature vary generally from about 36° C. to about 42° C.—too much for conventional sensors to be accurate. Thus, raw data from such sensors may provide misleading information regarding the quantity of drug remaining in an implantable infusion pump reservoir when the temperature of a patient fluctuates.
Accordingly, a need exists for devices and methods of accurately determining the amount of fluid in a reservoir and/or the fluid flow from a reservoir to help maintain proper operation of an implantable infusion pump.